Geoscience Reference
In-Depth Information
spectrum defines this alignment. A phase of zero means that the component sine wave has a maximum
at zero time, a phase of 90
◦
means a zero-crossing at zero time, and a phase of 180
◦
means a
minimum at zero time. The phase spectrum is a plot of the phase of the component sine waves against
frequency.
Point bar
Where a river bends, the maximum flow velocity is close to the outer bank. At the inner bank, the
flow is less and sediment accumulates to form a point bar, that grows by lateral accretion. Typically
the deposits are sands, perhaps with some mud in the upper part.
Primary (reflection)
Signal that has travelled direct from source to reflecting interface to receiver, in contrast to multiples
(q.v.) with their more complex paths involving multiple bounces. The primaries carry the information
we need to create an image of subsurface structure.
Receiver gather
A collection of the traces recorded at a given receiver from all the various shot points that have been
recorded at that receiver. This is sometimes called a
common-receiver gather.
Creation of such a
gather involves re-ordering the traces recorded in the field, which will be organised as common-shot
gathers, i.e. the collection of traces recorded at all the different receivers from each shot.
Reflection coefficient
When a seismic wave of amplitude
A
is incident on an interface between two different media, it is in
general partly reflected and partly transmitted. If the amplitude of the reflected wave is
R
, then the
reflection coefficient is defined as the ratio
R
/
A
. A negative value indicates that the reflected wave is
180
◦
out of phase with the incident wave.
Refraction record
To correct land seismic traces for static shifts generated by lateral variations in the near-surface struc-
ture beneath shots and receivers, we need to know the thicknesses and velocities of the near-surface
layers. Usually, there is a low-velocity weathered layer near the surface, overlying a higher-velocity
layer. To investigate the thickness of the low-velocity layer, we can shoot refraction profiles. These
consist of long lines of receivers with a source at each end. With this geometry, the first arrival
at each receiver will usually be the head wave, often called a refraction. This travels along the
top of the high-velocity layer. Geometrically we can think of it as being predicted by Snell'sLaw,
which says that a ray travelling through the interface will be bent (refracted) away from the inter-
face normal. As the angle of incidence of the ray is increased, there will come a point, the critical
angle, at which the ray bending would make the ray in the second medium travel along the inter-
face. (The full theory that explains the amplitude of the head wave and its spatial variation is much
more complex.) By analysing the variation in head-wave arrival times from one receiver to another,
it is possible to map the changes in thickness of the low-velocity layer above the high-velocity
refractor.
Root-mean-square (rms) average
The root-mean-square (rms) average of a set of numbers is the square root of the arithmetic average
of their squares.
Seismic waves
The most important type of seismic wave is the P-wave, which is an ordinary sound wave. As it moves
through the rock, individual particles move backwards and forwards in a direction parallel to that of
wave propagation. The other type of wave that can exist in the body of the rock is the shear or S-wave,
